1. Technical Field
This invention relates to a fixing device and an image forming apparatus with the fixing device, and in particular, to a durable fixing device capable of fixing an unfixed image onto a recording medium and an image forming apparatus with the fixing device.
2. Related Art
In general, a fixing device used in an image forming apparatus, such as a copier, a facsimile, a printer, etc., includes a fixing roller heated by a heat source and a pressing roller brought in pressure contact with the fixing roller. A fixing nip is formed between the fixing roller and the pressing roller to allow a recording medium, on which an unfixed image is formed, to pass therethrough. When the recording medium passes through the fixing nip, the unfixed image on the recording medium is pressed and heated by the pressing roller and the fixing roller, and thereby fixed onto the recording medium.
In response to growing demand for energy efficiency and a shortening of the waiting time required to heat the fixing device up to a prescribed operating level (e.g. a warm-up time and a time to first print, or first-print time), a so-called on-demand type fixing device that employs an endless belt such as a thin film or the like instead of a heating roller has been widely adopted. The on-demand type-fixing device reduces a heat capacity and upgrades effectiveness of heat transfer to the recording medium, while shortening the waiting time.
Specifically, in this type of a fixing device, a nip-forming member contacts an inner circumferential surface of a fixing belt. A rotator (e.g., a pressing roller) acting as a driving source is pressed against the fixing nip-forming member via a fixing belt, thereby forming a fixing nip between the fixing belt and the rotator. The recording medium is subsequently conveyed into the fixing nip to fix an unfixed toner image onto the recording medium.
In such a fixing device, since the fixing belt is pressed against the fixing nip-forming member by the rotator and is moved with its inner circumferential surface contacting the fixing nip-forming member, the fixing belt and the fixing nip-forming member are easily worn out.
Further, when friction between the fixing belt and the fixing nip-forming member increases in the fixing device, driving motor a torque to drive the rotator accordingly increases, thereby causing the rotator to slip and be unable to drive the fixing belt in the fixing device. As a result, the recording medium passing through the fixing nip is subjected to unstable braking and is wrinkled.
Further, once the driving motor becomes unable to bear a load, it seizes up and stops rotating.
Further still, the number of pulses inputted to the driving motor does not correspond to an actual rotation number of the driving motor resulting in a loss of synchronism
Known systems insert a sliding pad retaining lubricant between an inner circumferential surface of a fixing belt and a nip-forming member to render the fixing belt to smoothly slide thereon.
For example, a porous resin fiber woven fabric or a porous resin member prepared by laminating a porous resin film on a surface of the porous plastic fiber woven fabric and silicone oil are conventionally employed as the sliding pad and the lubricant, respectively.
Accordingly, a sheet-like sliding member prepared by laminating a deformation prevention film that prevents deformation of a porous material on a non-sliding surface of the porous material and silicone oil are also conventionally employed as the sliding pad and the lubricant, respectively.
However, since vacancies in the conventional porous resin member and the sheet-like sliding member are crushed by pressure during image fixation, and accordingly, the lubricant is squeezed out, the lubricant is not retained for a long time. At the same time, the fixing belt type-fixing device needs to be steadily and constantly driven for a long time. Thus, the sliding pad needs to better lubricant retention.